1. Field of the Invention
The present invention relates to a brushless motor for use as a fan motor or the like having an internal circuit base.
2. Description of the Related Art
Generally, a radial gap type brushless motor for use as a fan motor or the like has a housing having a supporting plate on its bottom, a cylindrical bearing tower projecting from a central portion of the supporting plate, a stator fixed on a circumferential surface of the bearing tower, a shaft rotatably supported by a bearing inside the bearing tower, a rotor fixed to this shaft and a cylindrical rotor magnet attached to the rotor so as to face a circumferential surface of the stator with a radial gap formed therebetween, and a circuit base on which electronic parts are mounted and which may include a device for detecting the rotational position of the rotor and a circuit for controlling a current for energizing the stator. This circuit base is disposed between the supporting plate and the stator, and coils on the stator are electrically connected to the circuit thereon.
In this kind of brushless motor is a fan motor, an impeller is formed integrally on the rotor. In such a case, a cup-like rotor holder having vanes is molded on the shaft by insert molding, the rotor yoke is then fixed on an inner cylindrical wall surface of the rotor holder, and the rotor magnet is fixed on an inner circumferential surface of the rotor yoke.
Brushless motors having such a construction are disclosed in the specifications of U.S. Pat. Nos. 5,013,957 and 5,343,104 and other documents.
This type of brushless motor, however, has a drawback because there is a need to mount predetermined circuit parts, including a position detecting device, control circuit parts, on the circuit base disposed between the stator and the supporting plate. The circuit base requires a certain mounting area even if the circuit component parts are mounted at a high density. Therefore, if this type of brushless motor is designed to be reduced in size, it is necessary to reduce the size of each component part to achieve a reduction in overall size. However, it is very difficult to do so while maintaining a certain mounting area of the circuit base. In particular, in the case of the fan motors disclosed in U.S. Pat. Nos. 5,013,957 and 5,343,104 and so on, the size of the circuit base cannot be reduced while the outside diameters of the stator and the impeller are reduced for a reduction in overall size, so that the circuit base protrudes outwardly beyond the rotor to partially close the passage for air flow caused by the impeller. That is, it is essentially impossible to manufacture a smaller fan motor by this construction.
The above-described brushless motor, if it is a fan motor, requires manufacturing steps of molding the cup-like rotor holder on the shaft by insert molding and then fixing the rotor yoke and the rotor magnet successively to form the rotor. Therefore, the assembly process is complicated and troublesome operations are required.